Protein powders come in various forms. The three commons ones are whey, soy, and casein. Whey is the most commonly used because it is water-soluble milk protein. It is also a complete protein, so it has all those advantages. A complete proteins contain all nine of the amino acids necessary for human need. People who are vegan may prefer soy protein although its taste is sometimes considered to be more unpleasant, and it does not dissolve in water.